Do you want to get the user Name and Password from Server (or) you want to send the user Name and password to the server? If you want to get data from Server use Http operation GET and if you need to send data to server use POST
Here are few links which you might find useful
http://www.codeproject.com/Tips/765853/Retrieve-Data-From-WCF-Webservice-using-WCF-REST
http://jonathanvanderoost.com/2014/02/08/create-restful-wcf-service-api-with-json-receive-generic-json/